1. Field of the Invention
This invention relates to thermal ink jet printing devices and, more particularly, to improved printheads which have a recirculating cooling system to regulate temperature that is located directly under the heating elements which expel ink droplets on demand.
2. Description of the Prior Art
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electrical pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet.
It is well known that print quality is affected as the device heats up. In particular, if the device heats up too high (e.g., during extended high density printing), then it tends to lose prime, and one or more ink channels of the printhead cease to expel droplets. A less catastrophic defect, but still one that degrades print quality, is the increase in printed spot or pixel size as a function of device temperature. Many of the prior art devices incorporate a heat sink of sufficient thermal mass and of low enough thermal resistance that the device temperature does not rise excessively. For one example of a thermal ink jet printhead having a heat sink, refer to U.S. Pat. No. 4,831,390 to Deshpande et al. This approach has eliminated the catastrophic printing failure mode. However, to lower the thermal resistance to the heat sink sufficiently that there is no appreciable device temperture rise in the time scale of a carriage translation in one direction across the paper, it may be necessary to take packaging approaches which would increase the cost or otherwise constrain the printer design in an undesirable way. The problem is expected to be worse as array size is increased.
U.S. Pat. No. Re. 32,572 to Hawkins et al discloses a thermal ink jet printhead and method of fabrication. A plurality of printheads are concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one substrate surface and etching corresponding sets of grooves which may serve as ink channels with a common reservoir in the surface of a silicon wafer. The wafer and substrate are aligned and bonded together so that each channel has a heating element. The individual printheads are obtained by milling away the unwanted silicon material in the etched wafer to expose the addressing electrode terminals on the substrate and then the bonded structure is diced into a plurality of separate printheads.
U.S. Pat. No. 4,638,337 to Torpey et al discloses an improved thermal ink jet printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blowout, which causes ingestion of air and interrupts the printhead operation whenever this event occurs. In this patent, a thick film organic structure such as Riston.RTM. is interposed between the heater plate and the channel plate. The purpose of this layer is to have recesses formed therein directly above the heating elements to contain the bubble which is formed over the heating element to enable an increase in droplet velocity without the occurrent of vapor blowout.
U.S. Pat. No. 4,774,530 to Hawkins discloses a two part printhead comprising a channel plate and a heater plate similar to the printhead of U.S. Pat. No. 4,638,337 to Torpey et al, but having a second pit or groove in the thick film layer sandwiched between the channel and heater plates. This second groove in the thick film layer is located so that the ink may flow from the reservoir to the ink channels even though the etched channel grooves do not connect to the adjacent etched reservoir recess.
U.S. Pat. No. 4,704,620 to Ichihashi et al discloses a multi-color ink jet printer having a plurality of printheads, one for each color, and each printhead has a temperature sensor. A temperature control system activates one or both fans on each side of the spaced, parallel printheads depending on the temperature of the individual printheads.